Add unit tests for ReverseStringUsingStack

Author: navaneedan07

src/test/java/com/thealgorithms/stacks/ReverseStringUsingStackTest.java

@@ -37,6 +37,7 @@ void testWhitespaceHandling() {
     }
 
     @Test
-    void testNullInput() { assertThrows(NullPointerException.class, () -> { ReverseStringUsingStack.reverse(null); });
+    void testNullInput() {
+        assertThrows(NullPointerException.class, () -> { ReverseStringUsingStack.reverse(null); });
     }
 }